Directional audio systems work by spatially filtering received (or transmitted) audio so that sounds received (transmitted) along the steering direction are amplified and sounds received (transmitted) along other directions are reduced. The reception or transmission of sound along a particular spatial direction is a classic but difficult audio engineering problem. One means of accomplishing this is by use of a directional array of transducers. It is well known by those skilled in the art that a collection of transducers can be treated together as an array to be combined in engineered ways to spatially filter (either when transmitting or receiving) directional sounds at the particular location of the array over time. The classic means of spatial filtering consists simply of manipulating the constructive and destructive interference pattern of the various sounds that pass through the array using some engineered combination of transducer types, array geometry, time delays, phase delays, frequency filtering, amplitude filtering, and temporal filtering to create a directional interference (a.k.a. directivity) pattern. Applications for the remote transmission or reception of audio require operation in many different, challenging environments including not only long distances, but also reverberant and noisy acoustic spaces and scenarios where size, weight, and power restrictions are severe. Limited scenarios have been addressed by prior devices, such as hands-free directional microphones for automobiles, small microphone arrays for computer workstations, hearing aids, modular microphone arrays, and loudspeaker arrays. However, none of these prior devices simultaneously solves the problems inherent in many common scenarios for directional audio systems—namely, size, weight, power, consistent directionality, scalability, and bi-directionality. By scalability, it is meant the characteristic to expand the size (e.g. physical aperture, number of transducers, etc.) of a directional audio system in an efficient manner to increase its effectiveness in or appropriateness for the application without compromising the simplicity, noise performance, power consumption, or architecture. By consistent directionality, it is meant the characteristic of a directional audio system that its directionality not vary significantly over the frequency range of interest. By bi-directionality, it is meant the characteristic of a directional audio system that its architecture can be used to transmit or receive audio, depending on the selection of the type of transducer. Therefore, significant problems remain for prior devices to function effectively in more general cases.
Traditional directional audio arrays by definition selectively receive or transmit sounds situated directly in-line with their (on-axis) look direction and have the ability to reduce sounds received from or transmitted to other (off-axis) directions. A transducer array can be used as a directional audio system and consists of, in its simplest form, a plurality of transducers with appropriate processing of the audio signals from or to the transducers so as to accomplish the formation of a directivity pattern.
Transducer arrays of this type, which use direct summation of the signals at the array of transducers, produce a directivity (i.e. width of the main lobe of the directivity pattern) which depends on the frequency. The directivity also generally depends on the effective dimensions of the array and the acoustic wavelength at the inspected frequency relative to that effective dimension. Therefore, at low frequencies a lesser degree of directivity is achieved and the directivity increases with the frequency.
The lowest wavelength at which a transducer array can provide a certain degree of directivity is dependent on the overall dimensions of the array. The highest frequency at which the directivity pattern does not exhibit spatial aliasing (which causes loss of directional characteristics at high frequencies) depends on the distance between the transducers in the array.
A significant side lobe is generally an undesirable characteristic of an array. In most applications, it is desirable to have minimum side lobes and a highly directional main lobe (traditionally defined as having a beam width of less than or equal to 25 degrees). Side lobes are determined by the number and geometrical configuration of the transducers in the array. It is known by those skilled in the art that if an axis of symmetry can be drawn through the geometrical configuration of the array of transducers, higher side lobes at some or all frequencies will result.
Increasing the size of an array has traditionally been accomplished by appending a duplicate of some, or all, of the existing array, including its spacing. Regardless of which traditional transducer configuration is used (e.g. equal, logarithmic, random, etc.), simply duplicating the existing configuration and appending it to the existing array in the same orientation will automatically result in an axis of symmetry and, hence, increased side lobes for the larger resulting array.